Screening apparatuses conventionally employed for screening chips are categorized into flat screens, disc screens and roller screens.
The purpose and benefits of screening are discussed in several books and patent publications related to the art. The task performed by screening apparatuses is to sort chips into three categories:
accept chips PA1 overlong and overthick chips suited to recycling PA1 reject portion or fines. PA1 only a portion of overlong and overthick chips can enter the portion of accepts, PA1 only a small portion of good chips lands in the portion routed to further processing, that is, rechipping, PA1 reject fines are separated quickly with only a small portion of accepts in its, thus permitting the use of a small-capacity fines screen.
Disc screens known in the art are fast and effective in screening away fines, but handicapped by the problem that, in addition to passing accepts, they also let through a portion of overthick and overlong chips particularly at the end stage of the screen, whereby a major portion of accepts has already passed the screen. Moreover, a disc screen produces some fines and pins as a result of the aggressive shaping of the discs. Simply, the major benefit of a disc screen is its extremely fast fines screening capability.
The chief problem of a roller screen is that the fed blanket of chips at high throughput is carried over the rollers, whereby fines follow a long way along toward the end stage of the screen. Thus, a roller screen is principally capable of effectively screening away oversize chips, but accepts and fines are largely screened simultaneously.
Consequently, fines must be screened away from accept chips at the second partial screening stage. The dimensioning of this so-called fines screen stage and the amount of chips to be screened are dependent on the length of the screen section capable of screening away a major portion of fines contained in the main chips flow.